1. Field of the Invention
The present invention relates to a radio terminal, a radio base station and a radio communication method capable of measuring transmission delay, jitter or the like.
2. Description of the Related Art
Heretofore, a communication quality measurement technique using an IP (Internet Protocol) packet has been widely used for measuring a quality of communications in a communication path between communication devices in a communication system (hereinafter, simply referred to as a “communication quality”) Here, the communication quality includes transmission delay, jitter or the like. The transmission delay is the amount of time equivalent to a time lag between the transmission time and the reception time of a packet. The jitter is a value indicating the amount of variation (fluctuation) in the transmission delay.
For example, the transmission delay and the jitter are measured by using a timestamp added to an IP packet. With such a communication quality measurement technique, a communication device measures a communication quality and performs communication control adaptive to the communication quality (hereinafter referred to as “adaptive communication control”). Such adaptive communication control includes adaptive buffer control for adaptively controlling the size of a receiving buffer absorbing jitter (what is called a jitter buffer).
In addition, in recent years, along with advancement in radio communication techniques, a radio terminal such as a cellular phone terminal has become connectable to multiple radio communication networks using different radio communication schemes. When a radio terminal executes handover from a certain radio communication network to another radio communication network, the radio terminal acquires a new IP address (care of address, for example) (refer to Japanese Patent Application Publication No. 2007-311952).
In the meantime, the header of an IP packet normally includes a source IP address and a destination IP address. For this reason, in order to transmit and receive an IP packet, each of communication devices on the transmission side and the receiving side needs to acquire an IP address.
In the conventional communication quality measurement technique, the communication quality is measured by using an IP packet. Accordingly, when a radio terminal is assigned a new IP address at the time of handover, the following problem occurs.
Specifically, until acquiring a new IP address, the radio terminal cannot transmit and receive an IP packet to and from a correspondent node (CN) via a target radio communication network (target radio base station) for handover. For this reason, the communication quality between the radio terminal and the CN via the target radio base station for handover cannot be measured. This leads to a problem that the adaptive communication control cannot be started until the radio terminal acquires the new IP address.
In particular, transmission delay and jitter in a radio communication path between the radio terminal and the radio base station are dominant in the transmission quality in the communication path between the radio terminal and the CN. Thus, it is preferable to make measurable the communication quality such as the transmission delay and the jitter in the radio communication path (hereinafter, referred to as a “radio communication quality”).